Function applyFaststart

packages/engine/src/services/chunkEncoder.ts:804–843

(
  inputPath: string,
  outputPath: string,
  signal?: AbortSignal,
  config?: Partial<Pick<EngineConfig, "ffmpegProcessTimeout">>,
  fps?: Fps,
)

}

export async function applyFaststart(
  inputPath: string,
  outputPath: string,
  signal?: AbortSignal,
  config?: Partial<Pick<EngineConfig, "ffmpegProcessTimeout">>,
  fps?: Fps,
): Promise<MuxResult> {
  // faststart is MP4-only (moves moov atom to file start for streaming).
  // WebM and MOV don't need it — skip the re-mux.
  if (outputPath.endsWith(".webm") || outputPath.endsWith(".mov")) {
    if (inputPath !== outputPath) copyFileSync(inputPath, outputPath);
    return { success: true, outputPath, durationMs: 0 };
  }
  const args = ["-i", inputPath, "-c", "copy", "-movflags", "+faststart"];
  if (fps !== undefined) {
    // Set the exact output framerate so the final remux doesn't PTS-average
    // a fractional rational like `360000/12001` instead of `30/1` into the
    // output container metadata. `-c copy` is retained; no re-encode.
    args.push("-r", fpsToFfmpegArg(fps));
  }
  args.push("-y", outputPath);

  const processTimeout = config?.ffmpegProcessTimeout ?? DEFAULT_CONFIG.ffmpegProcessTimeout;
  const result = await runFfmpeg(args, { signal, timeout: processTimeout });

  if (signal?.aborted) {
    return {
      success: false,
      outputPath,
      durationMs: result.durationMs,
      error: "FFmpeg faststart cancelled",
    };
  }
  return {
    success: result.success,
    outputPath,
    durationMs: result.durationMs,
    error: !result.success ? formatFfmpegError(result.exitCode, result.stderr) : undefined,
  };
}